1. Field of the Invention
The present invention relates to a computerized tomography apparatus. More specifically, the present invention relates to a computerized tomography apparatus which permits continuous scanning.
2. Description of the Related Art
In general, a computerized tomography (CT) apparatus performs three processes of scanning, image reconstruction and image display in time sequence. Projection data, acquired from many directions while an X-ray tube rotates or the X-ray tube and a detector array rotate together, is digitized, then subjected to preprocessing, such as calibration, and temporarily stored as raw data on large-volume storage, such as magnetic disk storage.
At the time of image reconstruction, the raw data is read out of the magnetic disk storage, then fed into a reconstruction unit via a memory. Tomography image data reconstructed in the reconstruction unit is stored on magnetic disk storage and transferred as a video signal to a cathode ray tube (CRT) monitor via a display memory.
The introduction of slip rings has enabled a continuous scan to be made. This continuous scan permits multi-directional projection data for a single slice or multiple slices to be obtained in time sequence. The multi-direction projection data is read from the magnetic disk storage into the reconstruction unit at arbitrary timing, then used for image reconstruction as described above. The time required for the image reconstruction is longer than the scanning time, and the time it takes to write to and access to the magnetic disk storage also is long. It is therefore impossible to display tomography images successively in real time like motion pictures while making a continuous scan.
The high-speed processing of image reconstruction was considered in recent years and is reaching the stage of practical use. Thus, the CT apparatus has been expected to display successively tomography images in real time while making a continuous scan. However, the long storage and access times of magnetic disk storage are serious obstacles to practical use of the high-speed processing of image reconstruction. Further, the wait state occurs irregularly in writing to and accessing magnetic disk storage. Thus, the time interval between a scan and the display of a tomography image varies irregularly, making it impossible to reproduce the internal movements of a section of a human body under examination.
Besides, the following problems arise in connection with the use of real-time X-ray CT apparatus in a clinical field. The first problem is that it takes a long time for an operator to place a slice or cross section of interest of a subject under examination in position by manually moving a top board on which the subject lies while watching a cross beam of light projected onto the subject from a projector. The second problem is that, when a scan that involves the movement of a top board (e.g., a helical scan, a multi-slice scan or the like) is suspended, it takes a long time to restart it. This is because a piece of work is required at the time of restart to put back the top board manually so that a slice of interest will be placed in the position at the time of suspension of the scan.